Anomalies¶
An anomaly map answers: is this unusual for this place at this time of year? 50 cm of snow is a lot in a valley in November and very little on a high pass in March. An anomaly accounts for that.
How it is calculated¶
For every terrain unit and every day of the season, the climatology holds three numbers from past years: the 5th percentile (q05), the median (q50) and the 95th percentile (q95). The anomaly is
so it is measured in units of the usual range (labelled IQR on the legend). It is clipped at ±3.
| Anomaly | Means |
|---|---|
| 0 | exactly the median for this day |
| within ±0.5 | normal: inside the band where about 90 % of past years fall |
| +1 | above normal by one full usual range |
| −1 | below normal by one full usual range |
| ±3 | extreme; the map shows anything beyond as ±3 |
Because the "normal" band is wide by design, most of the map normally reads as normal. Colour shows up where something is genuinely unusual.
Colours¶
For wet variables (SWE, snow depth, snowfall, precipitation, rainfall, runoff), blue means more than usual and red/brown means less. For the others (temperature, radiation, wind…) red means higher and blue lower. The legend always states which way round it is.
Where no anomaly is shown¶
Where a place almost never has snow on this date, the usual range is close to zero, and any trace of snow would look extreme. To avoid that, the anomaly is blanked where the climatological spread is below a floor (10 mm for SWE, 2 cm for snow depth). Glacier units are excluded too, because their snow builds up over years (see Known limitations).
Things to keep in mind¶
- The climatology is built from the same model run over past years, so a model bias that is the same every year cancels out in the anomaly. This is why anomalies are often more reliable than absolute amounts.
- The climatology period varies by region. See Domains and the board's Info tab.
- The season starts on 1 September in most regions. See Climatology and anomalies.